In these days, research and development are being vigorously made on various display elements. Especially, organic electric field light-emitting (EL) elements have been noted as promising display elements since they emit light with a high luminance at a low voltage. For example, there have been known light-emitting elements prepared by forming an organic thin film through vacuum deposition of an organic compound (Applied Physics Letters, vol. 51, p. 913, 1987). The light-emitting elements described in this literature, wherein tris(8-hydroxyquinolinato) aluminum complex (Alq) is used as an electron transporting material and is formed into a layered structure together with a hole transporting material (an amine compound), show a markedly improved light-emitting performance in comparison with conventional single-layered elements.
In recent years, it has been vigorously investigated to apply the organic EL elements to a color display or a white light source. However, in order to develop a high-performance display and a white light source, it is necessary to improve properties of each of a blue light-emitting element, a green light-emitting element and a red light-emitting element.
As means for improving the properties of light-emitting elements, there have been reported green light-emitting elements utilizing emission of light from an orthometalated iridium complex (Ir (ppy)3:Tris-Ortho-Metalated Complex of Iridium (III) with 2-phenylpyridine) (Applied Physics Letters, 75, 4 (1999)). This element shows an external quantum yield of 8%, which exceeds the conventionally believed upper limit as to an external quantum yield of 5%. However, since the color of emitted light is limited to green, it finds only limited applications to displays. Thus, it has been desired to develop elements capable of emitting other color than green with a high efficiency.